Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 161

5.9 Arctic general circulation
As in the Antarctic, the interactions between sea, ice, and land have an essential
role in determining the strength and variability of the general circulation in the
Arctic. As mentioned earlier, the major physical difference, however, is ice over
water in the Arctic polar region, the surrounding land masses, and the scattered
high elevation areas such as Greenland. Maxwell ( 1992 ) emphasizes the impor-
tance of open water (leads) within the ice pack, and the coastal impacts of land,
islands, and the sea channels in between. The atmospheric circulation provides
95% of the heat advection from lower latitudes into the Arctic Basin, with the
rest coming through the ocean (Bobylev et al. 2003 ).
The circulation patterns over the Arctic are controlled largely through the
strength and variations in the CPV. A deep, cold low extending through the mid-
troposphere to the lower stratosphere is a climatologically permanent fixture.
Geopotential heights at 500 hPa drop from 5800 m in the mid-latitudes to 5100m
over the pole (Maxwell 1992 ). Geographically, within the circumpolar vortex
there are three core troughs: over northern Canada and the Baffin Bay area (the
strongest); over the island of Novaya Zemlaya; and over the east coast of Siberia.
Vortex fluctuations are normally strongest in the Eurasian sector, and weakest
over North America (Frauenfeld and Davis 2003 ).
The AO/NAO variations explain a significant portion of the CPV fluctua-
tion pattern. Bobylev et al.( 2003 ) and Frauenfeld and Davis ( 2003 ) state that,
as the strength of the AO/NAO has increased, a more contracted and stronger
CPV creates stronger westerlies, and enhances warming in the Arctic. Other
impacts include stronger heat and moisture transport into the Basin, enhanced
convective activity leading to more frequent low pressure systems, and
changes in sea ice extent and thickness. Vortex circulation is strongest in
winter. This season also produces the greatest variations from mean tempera-
tures, largely due to warm air advection, but also caused by variations in
longwave radiation loss, and in the land-ocean-atmosphere interactions
(Bobylev et al. 2003 ). The intensity of the circumpolar vortex is enhanced
by winter cooling of the polar stratosphere. However, there is considerable
meandering of the vortex, and irregular breakdowns allow incursions from
lower latitudes.
5.10 Surface pressure and wind
Figure 5.11 provides a summary of surface pressure variations across the
Arctic Basin in summer and winter. The isobaric pattern shows higher pressure
( > 1021 hPa) in the eastern Arctic in winter. However the climate average
shown obscures a highly variable day-to-day situation. The Atlantic sector to
the North Pole is dominated by lower pressure, and, on average, strong pressure
gradients, which enhance the warm air incursions from this source. The control